Organic optoelectronic devices such as organic light emitting diodes, devices including photovoltaic cells, and displays such as organic thin film transistors must be encapsulated to protect their sensitive components from gases in air (mainly oxygen and/or moisture). Improper protection may cause deterioration in quality of the devices. In addition, this may cause occurrence of non-radial dark spots, which also leads to degradation of devices. In particular, in an organic light emitting diode, water vapor may cause degradation of the diode and deterioration in quality of an interface between an anode (or cathode) and an organic film.
Encapsulation may be typically achieved by bonding a glass cap to a display using specific adhesives, particularly adhesives having low water vapor permeability. Generally, to extend lifespan of devices, a solid moisture getter may be interposed between a substrate and a cap. Encapsulation using a cap is suitable for rigid devices, but is not suited for devices including a flexible support (for example, flexible displays).
Such an encapsulation technique is not feasible when a circuit of a substrate does not have a sufficient space as in a complementary metal-oxide semiconductor (CMOS) micro-display. Particularly, to achieve light weight, application of this technique must be avoided for a device having a large emission area.
In all cases for which encapsulation using a cap is unsuitable, “monolithic” encapsulation, i.e. encapsulation using a thin film having excellent oxygen barrier and water vapor barrier properties is generally used. Examples of most commonly used materials for monolithic encapsulation may include oxide dielectrics/nitrides, which are represented by SiOx, SiNx, SiOxNy, and AlxOy, deposited generally using chemical vapor deposition (CVD), and optionally using plasma-enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD). The aforementioned methods are preferable to physical vapor deposition such as sputtering, which is aggressive to organic semiconductors and thus causes formation of a film having difficulty in coating a protective film thereon due to defects such as pinholes produced in the deposited film. Deposited films obtained by plasma-enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD) have fewer defects than films obtained by physical vapor deposition and are very uniform. In other words, these two methods may provide excellent step coverage.
“To avoid association” of defects of an inorganic layer with other inorganic layers, there has been, for example, an effort to manufacture a multilayer of organic/inorganic/organic/inorganic layers, which is referred to as Barix. This method may reduce water vapor permeability to about 10−6 g/m2/day and thus provide sufficient lifespan to allow commercialization of organic light emitting diode displays.
Another typical example of multilayer encapsulation structures includes “NONON” by Philips Electronics, which is a multilayer including a nitride layer and an oxide layer alternately stacked one above another, such as SiNx/SiOx/SiNx/SiOx.
In this regard, U.S. Pat. No. 7,767,498 reported that waterproofing properties of about 10−6 g/m2/day were attained by repeatedly depositing about 5 acrylic organic layers and 5 inorganic layers through vacuum deposition. However, in the structure formed by stacking the organic layers, since the organic layers are composed of an organic material having no barrier properties, a cathode layer suffers from corrosion due to water penetration causing light emission failure, which results in deterioration in reliability. In addition, in this structure including 10 deposited layers, if the organic layer does not have a sufficient thickness, the organic layer suffers from deterioration in smoothness when deposited on the inorganic layer. Further, if aluminum oxide, which has good barrier characteristics, is used alone, pin-holes having been produced upon deposition continue to grow even when layer thickness is increased, thereby allowing easy penetration of water and oxygen therethrough. This may cause deterioration in adhesion between organic and inorganic layers and thus degradation of waterproofing properties.